Setting the Ownership of a Shared Mac Disk

| How-To

HDDs, SSDs or Flash drives connected to a Mac can have their ownership relaxed when the drive is intended to be removed and shared with others. Here’s how and why to change the setting.

First, note that the ownership of all files and folders is restricted on the boot volume, which is owned by root. If you’d like to verify this, you use do the following:

  1. Open the terminal app
  2. enter: cd /Volumes
  3. enter: ls -la

In my case, the internal boot drive, Altair, is owned by root. That’s necessary to enforce UNIX protocols for access and permissions by multiple users.

Terminal view of /Volumes

Terminal results for /Volumes

If you’d like to share an external drive with others, however, this UNIX protocol can get in the way when the drive is mounted on another Mac. The solution is to set a flag to ignore ownership concepts (but not permissions) on the drive. That flag is found in the Get Info (CMD-I) box for the drive at the very bottom. (Note you may have to authenticate with an admin password if the padlock has this setting locked.)

get Info for drive

Bottom Part of Get Info box for a drive

In fact, when you check this box, special UNIX flags are set such that whoever mounts the drive sees himself/herself as the owner. It’s pretty neat. Note, however, the originally set permissions for that owner remain intact. So if the (floating) owner has only read-only permission for a file, that will remain in force for anyone who mounts the drive. Of course, as owner (and possibly admin user) those permissions can be altered.

If the drive is the boot drive or a Time Machine drive, you won’t have the option to set this flag.

If the flag is not set, the drive will show up in the terminal as owned by root. If you set the flag, it will show up in the terminal as owned by you. (See the first screen shot above.) Note that if you’ve installed a backup version of Mac OS X on a drive and expect to boot from it, DO NOT SET THE FLAG.

In summary, if you have an external drive you expect to share with others or move from Mac to Mac, it’s helpful to set the “Ignore ownership on this volume” flag to make copying files to and from the drive seamless. Bootable drives should never have the flag set.

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Comments

vpndev

A few clarifying comments ...
1. ownership is enforced by default on “internal” volumes. If you have a Mac Pro with extra disks, ownership will be applied to them. Same is true if you have partitioned a single disk in, say, an iMac. You have one disk with two volumes, and ownership is enforced on both volumes.

2. Ownership is *always* enforced on the boot volume. Even if it’s an external disk. Don’t mess with permissions on any of the system files of a disk you want to boot from. If you do it often becomes unbootable very fast.

3. External disks seem to have the “ignore” setting by default. I haven’t checked what happens if you take a disk from external to internal, though. I think it becomes “checked” unless you’ve changed the default

Paul Bogle

Excellent info. Thanks.

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